| Summary: | A minimal genome and absent bacterial cell wall renders Mycoplasma hominis inherently resistant to most antimicrobials except lincosamides, tetracyclines and fluoroquinionlones. Often dismissed as a commensal (except where linked to preterm birth), it causes septic arthritis in immunodeficient patients and is increasingly associated with transplant failure (particularly lung) accompanying immunosuppression. We examined antimicrobial susceptibility (AST) on strains archived between 2005-2015 submitted to the Public Health England reference laboratory and determined the underlying mechanism of resistance by whole genome sequencing (WGS). Archived M. hominis strains included 32/115 from invasive infection (sepsis, CSF, peritoneal and pleural fluid) over the 10-year period (6.4% of all samples submitted between 2010-2015 were positive). No clindamycin resistance was detected, while two strains were resistant to moxifloxacin and levofloxacin (resistance mutations: S83L or E87G in gyrA and S81I or E84V in parC). One of these strains and 11 additional strains were tetracycline resistant, mediated by tet(M) carried within an integrative conjugative element (ICE) consistently integrated at the somatic rumA gene; however, the ICEs varied widely in 5-19 associated accessory genes. WGS analysis showed tet(M)-carrying strains were not clonal, refuting previous speculation that the ICE was broken and immobile. We found tet(M)-positive and -negative strains (including the multi-resistant 2015 strain) to be equally susceptible to tigecycline and josamycin, however, the British National Formulary does not include guidance for these. Continued M. hominis investigation and AST surveillance (especially immunocompromised patients) is warranted, and expansion of the limited therapeutics needs to be expanded in the UK.
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